This report focuses on corrosion challenges with using the austenitic stainless steel AISI 316L (UNS S31603) in marine atmosphere. AISI 316L has been frequently used in marine atmosphere for several decades due to the corrosion resistance, however over the recent years pitting and crevice corrosion attacks has been observed. The aim of this study is to investigate the effect of small changes (0.5 wt%) in molybdenum content on the corrosion properties, and to study pit propagation with focus on repassivation and the effect of cathode area. The effect of chloride concentration and temperature are included when evaluating the effect of molybdenum content.

In this study, the effect of molybdenum content was investigated by conducting experiments with AISI 316L and alloys with similar composition. Anodic cyclic potentiodynamic polarisation curves were recorded with 3 and 5 wt% sodium chloride solution at both room temperature and 35 +/- 2 degrees Celsius. Open circuit potential measurements and exposure in a salt spray chamber were conducted by using 5 wt% sodium chloride at 35 +/- 2 degrees Celsius. To study propagation of a single pit a method to obtain the potential at the pit opening and the galvanic current from a single pit were developed and conducted with different set ups regarding the cathode area.

The result from the anodic cyclic potentiodynamic polarisation implied that differences in alloying content affects the pitting potential where the temperature has a slightly greater effect than chloride concentration, while the repassivation potential is not significantly affected. Considering the obtained results from the experiments, 3 wt% molybdenum increases the corrosion resistance. For alloys containing below 2.5 wt% molybdenum small changes do not significantly affect the corrosion resistance if the nitrogen content increases sufficient. Increased nitrogen content appeared to retain the corrosion properties as the molybdenum content decreases.

The result from the artificial pit experiments indicated that the pits propagated continuously and did not repassivate in 6wt% FeCl3 when connected to an external cathode area. Both the cathode area and electrolyte composition were established to considerably affect propagation of a single pit. The pit size increases with cathode area in 6 wt% FeCl3, and the cathode efficiency decreases significantly in 5 wt% NaCl compared to 6 wt% FeCl3.